Soft lithography has emerged as an important technique for the preparation of patterned structures for a wide range of applications. Soft lithography involves forming a pattern on a substrate, applying an uncured resin over the patterned substrate, curing the resin, and removing the resin to obtain a pattern that is inverse to that on the substrate. One of the most widely utilized resin in soft lithography is polydimethylsiloxane (PDMS), due to its low cost and robust/versatile material characteristics.
However, PDMS has two main drawbacks that limit its use in certain applications. First, PDMS is easily swelled by many organic small molecules, such as an organic solvent. Hence, it is not well-suited for many applications that involve organic chemistry.
Second, a functionalized PDMS surface loses its functional groups over time. Without wishing to be bound by theory, the loss of reactive groups on the surface is thought to be due to (a) the internalization of the reactive groups into the bulk of the PDMS and (b) the migration of PDMS molecules from the bulk of PDMS to the surface to cover the functionalized surface. Hence, reaction is possible only within a short time after the surface treatment.
Many previous attempts in rendering PDMS surface have been unsatisfactory. For example, Roman et al., “Sol-Gel Modified Poly(dimethylsiloxane) Microfluidic Devices with High Electroosmotic Mobilities and Hydrophilic Channel Wall Characteristics,” Anal. Chem. (2005), Vol. 77, pp. 1414-1422 reports diffusion of metal oxide precursors throughout the bulk of the PDMS followed by a subsequent conversion to form metal oxides throughout the bulk of the PDMS. Roman and Culbertson, “Surface Engineering of Poly(dimethylsiloxane) Microfluidic Devices Using Transition Metal Sol-Gel Chemistry,” Langmuir, (2006), Vol. 22, pp. 4445-4451 also reported functionalization of microchannels on PDMS surfaces, flowing metal oxide precursors into the microchannels, followed by subsequent conversion to metal oxides. However, both of these techniques suffer from the problem that diffusion of the precursors into the PDMS causes significant swelling and potential deformation/warping of the patterned features present on the PDMS. Moreover, an interpenetrating network of PDMS and metal oxide is formed instead of the desired coating of metal oxide on the surface of the PDMS.
The above-identified problems are overcome herein and the patent application provides a coating on the PDMS surface that is resistant to organic solvents. The coating can be further treated to provide reactive groups that are stable for a prolonged period of time without significant deformation of the PDMS.